This invention generally relates to the field of radio receivers and, more particularly, to enhancing the audio quality in the radio receivers.
Improving the audio quality is of paramount importance to manufacturers of communication devices, because audio quality is easily noticeable by users of the radio receivers. Among other things, degraded audio quality can result from various types of disturbances, which distort radio frequency (RF) signals. These disturbances include fading, interference, and noise. A fading condition occurs when a RF signal transmitted to the radio receiver propagates through multiple transmission paths on an RF channel. As a result, the received signal is received from different directions and fluctuates according to the propagation properties of the RF channel. A common example of fading occurs when the radio receiver along a road, while passing objects that reflect radio waves in many directions, for example, buildings or telephone poles, etc. Interference, which is also exposed to fading fluctuations, occurs when other transmitted signals interfere with the received signal at the radio receiver. Noise, which is caused by thermal conditions, has a constant spectral density. One or a combination of fading, interference, or noise can adversely influence the audio quality at the radio receiver.
When the number of propagation paths is large, the impulse response of the RF channel has a Rayleigh distribution pattern. Therefore, the received signal, which is known as Rayleigh-faded signal, fluctuates both in phase and amplitude. A destructive condition reduces the amplitude of the received signal into fading dips, and a constructive condition will increase the amplitude. During the fading dips, the magnitude of a faded received signal may become small enough to cause a substantial momentary decrease in the signal-to-noise ratio of the radio receiver. As a result, phase variations of the received signal generate repetitive and annoying audio disturbances, which are known as audio clicks. The severity of the audio clicks is proportional to the number of fading dips. As the number of fading dips increases, the audio quality at the receiver worsens. The extent of audio quality degradation, therefore, depends on how severe the multipath propagation is together with the average received signal strength and the interference and/or noise levels.
Various noise reduction techniques have been used to control noise bursts and to reduce the overall noise level in radio receivers. Such techniques include, for example, syllabic companding, high frequency equalization, automatic gain control (AGC) circuits, noise blankers, and space diversity receivers. However, these techniques are generally ineffective in reducing the audio disturbances associated with fading conditions.
Conventional methods generally rely on the received signal strength or discriminator noise levels at the radio receiver for reducing audio clicks caused by fading. According to one method, an analog FM receiver reduces noise bursts and audio clicks by selectively attenuating demodulated audio signals in response to a control signal derived from one or a combination of the discriminator noise or received signal strength. To reduce the audio clicks, the audio signals are attenuated when the control signal exceeds a threshold. In another method, the audio clicks are eliminated by limiting the gain of a front end RF amplifier through which the faded signals are received. However, received signal strength and discriminator noise are not accurate parameters for detecting fading conditions. This is because the received signal strength and discriminator noise may be influenced by factors not associated with fading. As such, these conventional methods do not accurately detect fading conditions. For example, the received signal strength may decrease because of a large distance between the radio receiver and transmitter. Under this scenario, the conventional methods that rely on the received signal strength and discriminator noise may unnecessarily attenuate the audio or receiver gain, when no fading condition is present.
With the advent of powerful signal processors, many of today""s communication systems are becoming digital. In digital communication systems, analog audio signals are digitized and coded into digital audio data. The digital audio data are transmitted in bursts, with each burst consisting of a predefined number of data bits. Digital communication systems improve audio quality using various methods that conceal errors associated with digital audio processing. For example, one conventional audio processing method recovers missing audio data packets using a Low Delay Code Excited Linear Prediction (L-CELP) algorithm. Another audio processing method erases speech frames when an error in the coding of the digital audio data is encountered. Other methods conceal distorted audio by replacing it with an estimate of how the audio should look like at an instant in time. When processing the digital audio data, these audio processing techniques, however, do not take into account the fading conditions associated with the RF channel. As a result, these techniques do not detect and eliminate audio clicks associated with fading conditions. Therefore, there exists a need for a radio receiver that accurately detects and eliminates audio clicks associated with fading conditions.
The present invention that addresses this need is exemplified in a radio receiver that incorporates a method and apparatus for reducing audio clicks by accurately detecting fading conditions and reducing a faded signal""s phase variations, which cause the audio clicks.
Briefly, a method or apparatus according to the invention receives a faded signal and determines Its phase variations during a predefined time interval. If the phase variations exceed a phase variation threshold, for example, 2xcfx80 radian, the phase variations of the faded signal are reduced below a predefined level. According to one aspect of the invention, if the phase variations exceed the phase variation threshold, the strength of the faded signal is also determined as an additional factor for detecting fading conditions. Under this aspect, the phase variations of the faded signal are reduced below the predefined level only if the strength of the faded signal is below a signal strength threshold.
According to some of the more detailed features of the invention, the faded signal is processed to provide a complex baseband signal that includes inphase (I) and Quadrature (Q) components. The phase variations of the faded signal are determined based on the I and Q components of the baseband signal. In an exemplary embodiment, the phase variations of the faded signal are determined by sampling the complex baseband signal at a predefined sampling rate. Based on the sample""s corresponding I and Q components, a phase difference corresponding to each one of two consecutive samples is determined by subtracting the samples"" associated phases from each other. The phase differences of a predefined number of consecutive samples are summed to provide a phase variation associated with a baseband vector. The audio clicks are suppressed by reducing the phase variation of the baseband vector below the predefined level, if such phase variation exceeds the phase variation threshold. Preferably, the predefined level below which the phase variation is reduced is 2xcfx80 radian. In an exemplary arrangement, the phase variation is reduced by dividing the phase associated with each sample by an integer, such as 2, 4, etc.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings.